Electrical device



m5. s, 1938. J, B, HARLEY 2,107,752

ELECTRICAL DEVICE Filed Oct. 20, 1934 INVENTOR' JB .HARLEV ATTO NEVPatented Feb. 8, 1938 UNITED STATES PATENT 'OFFICE Y TelephoneLaboratories,

Incorporated, New

York, N. Y., a corporation of New York Application October 20, 1934,Serial No. 749,156

Claims.

This invention relates to electric devices and particularly to deviceshaving elements mounted so as to have a natural frequency of vibration.

The object of the invention is to compensate 5 for the inherentvariation in sensitivity of these devices due to their natural frequencyof vibration.

' A feature of the invention is an element having the same naturalfrequency of vibration as the moving elements of the device connected sothat the inherent increase in impedance of this element will counteractthe increase in sensitivity of the moving elements.

Another feature of the invention is an impedance element connected inshunt with the supply circuit which causes the moving elements to move.

In many known electric meters and devices for controlling light, such asoscillograph, light 20 valves, etc., the moving elements are mounted toform a mechanical vibrating system having mass, elasticity andstifl'ness. Such a mechanical system will have one or more naturalfrequencies of vibration, determined by the mechanical constants of thesystem. For frequencies approaching the resonant frequency of thesystem, the sensitivity or amplitude of response for given powerincreases rapidly and, at the same time, the motional'lmpedance of thedevice also increases rapidly.

The invention may conveniently be described as applied to a light valvefor film sound recording.

The recording elements of the light valve may be vibrated by electriccurrents flowing in the element itself, as in U. S. Patent 1,852,774,April 5, 1932 to E. W. Gent and L. M. Potts and U. S. Patent 1,914,186,June 13, 1933 to E. C. Wente, or by currents flowing in a moving coilattached to the element as in U. S. Patent 1,836,558, December 15, 1931to R. J. Sherman; or by currents flowing in a fixed coil influencing theelements as in U. S. Patent 1,794,513, March 3, 1931 to 0. O. Ceccarini.For convenience of description, only the flrst type of light valve willbe illustrated although it will be apparent to those skilled in the artthat the invention is not limited to the specific type of light valveillustrated but is equally applicable to the other types mentioned andto oscillographs and many electric meters.

In accordance with the invention, a dummy light valve, mechanicallysimilar to the recording light valve, is placed in series with therecording light valve in the electrical circuit supplying power to therecording light valve. The

dummy light valve does not, in any way, aifect the recording light beam.When the frequency of the currents to be recorded increases, themotional'impedance and the electrical impedance of the dummy light valveincrease. As the dum- 5 my light valve is in series with the recordinglight valve, the increase in the electrical impedance of. the dummylight valve will decrease the current actuating the recording elementsand thus compensate for the increased sensitivity of 10 the recordingelements.

To further aid in controlling the sensitivity, a fixed impedance may beconnected in parallel relation with the portion of circuit actuating therecording elements. When the frequency of the 15 currents increases, theelectrical impedance of the recording light valve also increases and anincreasing amount of the signal currents will flow through the fixedimpedance, thus variably decreasing the currents actuating the lightvalve.

The drawing diagrammatically shows an embodiment of the invention.

The light valve has a magnet I having parallel pole faces pierced byaligned apertures 2, 3. The magnet I may be a permanent magnet or anelectromagnet and the pole faces may be formed as part of the magnet ormay be a separate element formed of magnetic material. A flexibleconducting ribbon 4 is stretched from the support 5 over the insulatingbridges 6 and 1 around the insulating support 8 back over the bridges 6and l to the support 9. The two portions of the ribbon 4 are so disposedas to define a light transmitting slot aligned with the apertures 2, ,3in the magnet I. A similar ribbon is also 3 stretched from the supportl0 over the bridges 6, i to the support ll, thence back over the bridges6, l to the support l2. The ribbon stretched from support In to supporti2 may be a portion of the ribbon 4, as shown, or may be a separateribbon having similar mechanical properties. When both ribbons are ofthe same length and dimensions, both ribbons may be tensioned by asingle spring l3 attached to the movable supports 8, II. If separateribbons are used they may be tensioned by separate springs attachedrespectively to the supports 8 and II.

The specific details of construction of the light valve are notimportant and may be varied without departing from the invention. Thelight valve may, for example, be constructed similarly to the lightvalve shown in U. S. Patent 1,852,774, April 5, 1932 to E. W. Gent andL. M. Potts. Light valves of similar type are described in detail in anarticle The Principles of the Light 55 Valve by T. E. Shea, W. Herriottand W. R. Goehner, published in the Journal of the Society of MotionPicture Engineers, vol. XVIII, pp. 697-730, June, 1932.

The active light valve and the dummy light valve will normally be tunedto the same frequency of mechanical resonance. A small error in tuningwill not render the valve inoperative but will merely reduce theeffectiveness of the compensation.

Light from the source I4 is focussed by the lens system Hi to illuminatethe slit defined by the ribbon 4 in the aperture 2. This illuminatedslit is imaged by the lens IS on the constantly moving photographic filml1.

Signal currents from any suitable source, such as a microphone, pick-up,etc., are amplified in the amplifier l8 and induce an electromotiveforce in the secondary winding of the transformer l9. Current flows fromtransformer iii to support 5, through the four sections of the ribbon 4to support i2, thence back to transformer Ill. The current flowing inthe ribbon between supports 5 and 9 causes the sections of the ribbon tooscillate, thus varying the exposure of the film H in accordance withthe signal currents. An impedance 20 may also be connected from support5 to support 9 in parallel relation with the active light valve ribbon.This impedance may be of the same order as the impedance of the activeribbon at low frequencies. The impedance 20 may be non-inductive or mayhave an inductive component resonating with the capacity of the lightvalve at some frequency less than resonance.

In a mechanically tuned light valve, the impedance of the movingelements increases very rapidly as the frequency of the applied currentapproaches the frequency of mechanical resonance. Also, the sensitivityincreases, that is, the current required to produce any given deflectiondecreases rapidly when approaching the frequency of mechanicalresonance. In the present invention, when approaching the resonantfrequency, the electrical impedance of the ribbon between supports l andI2 increases rapidly thus reducing the current flowing in the ribbonbetween supports and 9 and the amplitude of oscillation which would beproduced.

At low frequencies, the signal current will be divided between theimpedance 20 and the ribbon stretched between the supports 5 and 9. Forconvenience, the current may be considered as equally divided, althoughother proportions may obviously be used. Now, when the frequency of thesignal current approaches the frequency of mechanical resonance of theribbon 4, the electrical impedance of the ribbon 4 will increaserapidly. As the impedance 20 does not change, a

smaller proportion of the current will flow through the ribbon 4 and theamplitude of oscillation will be lessened.

Thus, the portion of the ribbon between the supports l0 and i2, and theimpedance 20, cooperate to cause a reduction in the current flowing inthe ribbon between the supports 5 and 9, which reduction increasesrapidly when approaching the resonant frequency. The sensitivity of thelight valve is thus made more nearly uniform throughout an extendedrange.

What is claimed is:

1. In a light valve, in combination a magnetic structure having alignedapertures, an electrical conductor arranged to form a light transmittingslit controlling the transmission of a beam of light passing throughsaid apertures, a second electrical conductor arranged out of alignmentwith said apertures and ineffective to control the transmission of saidlight, means for supporting both said conductors in the magnetic fieldof said structure, means for tensioning both said conductors to havesubstantially the same frequency of mechanical resonance, and means forsupplying signal currents to both said conductors in serial relationshipto cause both said conductors to vibrate substantially in accordancewith the signal currents whereby the effects of mechanical resonance onthe control of said light beam aresubstantially reduced.

2. The combination in claim 1 with an electrical conductor of lowimpedance connected in parallel relationship with the ends of theconductor controlling the light.

3. In a light valve, in combination a magnetic structure having alignedapertures, a looped conducting ribbon defining a light transmitting slitaligned with said apertures, a similar looped conducting ribbon arrangedto be out of alignment with said apertures, means for supporting bothsaid ribbons in the magnetic field of said structure under tension so asto have substantially the same frequency of mechanical resonance, andmeans for supplying signal currents to both said ribbons in serialrelationship to cause both said ribbons to vibrate, the first ribbonvarying the light transmitting slit in accordance with the signalcurrents, the second ribbon presenting a variable impedance whereby theefiects of mechanical resonance on the motion of the first ribbon aresubstantially reduced.

4. The combination in claim 3 in which the sides of said loops areparallel.

5. The combination in claim 3 with an electrical impedance connected inparallel relationship with the ends of the ribbon defining the slit.

JOHN B. HARLEY.

